A Fourier transform infrared spectrophotometer (FTIR) has a two-beam interferometer, a representative of which is a Michelson interferometer including a beam splitter, fixed mirror and movable mirror. The movable mirror is made to move so as to change the optical path length difference (or phase difference) between two light beams and thereby generate infrared interference light whose amplitude (intensity) changes (i.e. the so-called “interferogram”). This infrared interference light is cast into or onto a sample, and the intensity of the light transmitted through or reflected by the sample is detected with a photodetector to obtain an interferogram of the transmitted or reflected light. By Fourier-transforming this interferogram, a spectrum (spectral characteristics) of the transmitted or reflected light can be obtained. This spectrum can be represented on a coordinate system which, for example, has a horizontal axis indicating the wavelength (or wavenumber) and a vertical axis indicating the intensity (e.g. absorbance or transmittance). A spectrum which covers a predetermined range of wavelengths can be obtained from an interferogram generated by one scan of a predetermined distance with the movable mirror (see Patent Literature 1).
Acquisition of exact and highly reproducible spectrum data requires accurate control of the timing of the measurement of the intensity of the transmitted or reflected light of the infrared interference light in the photodetector (this intensity is hereinafter called the “infrared interference light intensity”) and the moving position of the movable mirror. In other words, it is necessary to accurately control the optical path length difference between the two light beams. To this end, the two-beam interferometer normally includes a control interferometer for generating a signal for the sampling of the data of the infrared interference light intensity in addition to the main interferometer for the acquisition of the interferogram. The control interferometer generates laser interference light using a monochromatic light source (laser light source) as well as the beam splitter, fixed mirror, movable mirror and other elements which are shared with the main interference. A mirror is placed in an optical path of the main interferometer to extract the laser interference light from the optical path. The extracted light is introduced into a laser detector. Making the movable mirror move at a fixed speed causes a sinusoidal change in the intensity of the laser interference light at a fixed frequency. This sinusoidal wave is detected as a laser interference fringe signal. Based on this interference fringe signal, a signal for the data sampling is generated.
The interference fringe signal is normally processed as a voltage signal having an amplitude in both the plus and minus directions from a reference potential. The level of the voltage signal detected under the condition that the optical path length difference is zero is defined as the reference potential. The infrared interference light intensity is measured at the timing of an upward zero-crossing of the voltage signal (a point in time at which the signal rises above the reference potential) and/or the timing of a downward zero-crossing (a point in time at which the signal falls below the reference potential). A helium-neon (He—Ne) laser light source with an oscillation frequency of 632.8 nm is commonly used as the laser light source for the control interferometer in a FTIR. In that case, the infrared interference light intensity is measured at each point in time where the movable mirror reaches a position where the optical path length difference between the two light beams of the control interferometer equals an integer multiple of 632.8 nm or 316.4 nm.